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        <h3 id="内存简介"><a href="#内存简介" class="headerlink" title="内存简介"></a>内存简介</h3><p><img src="/xieyuanhui/2019/05/27/Java内存模型/img1.jpg" alt></p>
<p>32位处理器：$2^{32}$的可寻址范围</p>
<p>64位处理器：$2^{64}$的可寻址范围</p>
<h4 id="地址空间的划分"><a href="#地址空间的划分" class="headerlink" title="地址空间的划分"></a>地址空间的划分</h4><ul>
<li>内核空间</li>
<li>用户空间</li>
</ul>
<p><img src="/xieyuanhui/2019/05/27/Java内存模型/img2.jpg" alt></p>
<h3 id="JVM内存模型—JDK8"><a href="#JVM内存模型—JDK8" class="headerlink" title="JVM内存模型—JDK8"></a>JVM内存模型—JDK8</h3><p><img src="/xieyuanhui/2019/05/27/Java内存模型/img3.jpg" alt></p>
<p>线程私有：程序计数器、虚拟机栈、本地方法栈</p>
<p>线程共享：MetaSpace、Java堆</p>
<h4 id="程序计数器"><a href="#程序计数器" class="headerlink" title="程序计数器"></a>程序计数器</h4><blockquote>
<p>当前线程所执行的字节码行号指示器（逻辑）</p>
<p>改变计数器的值来选取下一条需要执行的字节码指令</p>
<p>和线程是一对一的关系即“线程私有”</p>
<p>对Java方法计数，如果是Native方法则计数器为Undefined</p>
<p>不会发生内存泄漏</p>
</blockquote>
<h4 id="Java虚拟机栈（Stack）"><a href="#Java虚拟机栈（Stack）" class="headerlink" title="Java虚拟机栈（Stack）"></a>Java虚拟机栈（Stack）</h4><p><img src="/xieyuanhui/2019/05/27/Java内存模型/img4.jpg" alt></p>
<blockquote>
<p>Java方法执行的内存模型</p>
<p>包含多个栈帧</p>
</blockquote>
<h5 id="局部变量表和操作数栈"><a href="#局部变量表和操作数栈" class="headerlink" title="局部变量表和操作数栈"></a>局部变量表和操作数栈</h5><blockquote>
<p>局部变量表：包含方法执行过程中的所有变量</p>
<p>操作数栈：入栈、出栈、复制、交换、产生消费变量</p>
</blockquote>
<p>下面用一个例子来看看局部变量表和操作数栈：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> com.imooc.interview.jvm.model;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ByteCodeSample</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">int</span> <span class="title">add</span><span class="params">(<span class="keyword">int</span> a, <span class="keyword">int</span> b)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">int</span> c = <span class="number">0</span>;</span><br><span class="line">        c = a + b;</span><br><span class="line">        <span class="keyword">return</span> c;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>执行 javac ByteCodeSample.java 得到 ByteCodeSample.class 文件，执行 javap -verbose ByteCodeSample.class 得到机器码指令：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br></pre></td><td class="code"><pre><span class="line">Classfile /E:/Projects/IDEAProjects/imooc-code/src/com/imooc/interview/jvm/model/ByteCodeSample.class</span><br><span class="line">Last modified <span class="number">2019</span>-<span class="number">5</span>-<span class="number">27</span>; size <span class="number">300</span> bytes</span><br><span class="line">        MD5 checksum a4bce828e4b4937c0ba585ed7635e5ba</span><br><span class="line">        Compiled from <span class="string">"ByteCodeSample.java"</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">com</span>.<span class="title">imooc</span>.<span class="title">interview</span>.<span class="title">jvm</span>.<span class="title">model</span>.<span class="title">ByteCodeSample</span></span></span><br><span class="line">        minor version: 0</span><br><span class="line">        major version: <span class="number">52</span></span><br><span class="line">        flags: ACC_PUBLIC, ACC_SUPER</span><br><span class="line">        Constant pool:</span><br><span class="line">        #1 = Methodref          #3.#12         // java/lang/Object."&lt;init&gt;":()V</span><br><span class="line">        #2 = Class              #13            // com/imooc/interview/jvm/model/ByteCodeSample</span><br><span class="line">        #3 = Class              #14            // java/lang/Object</span><br><span class="line">        #4 = Utf8               &lt;init&gt;</span><br><span class="line">   #5 = Utf8               ()V</span><br><span class="line">           #6 = Utf8               Code</span><br><span class="line">           #7 = Utf8               LineNumberTable</span><br><span class="line">           #8 = Utf8               add</span><br><span class="line">           #9 = Utf8               (II)I</span><br><span class="line">           #10 = Utf8               SourceFile</span><br><span class="line">           #11 = Utf8               ByteCodeSample.java</span><br><span class="line">           #12 = NameAndType        #4:#5          // "&lt;init&gt;":()V</span><br><span class="line">           #13 = Utf8               com/imooc/interview/jvm/model/ByteCodeSample</span><br><span class="line">           #14 = Utf8               java/lang/Object</span><br><span class="line">           &#123;</span><br><span class="line"><span class="keyword">public</span> com.imooc.interview.jvm.model.ByteCodeSample();</span><br><span class="line">        descriptor: ()V</span><br><span class="line">        flags: ACC_PUBLIC</span><br><span class="line">        Code:</span><br><span class="line">        stack=<span class="number">1</span>, locals=<span class="number">1</span>, args_size=<span class="number">1</span></span><br><span class="line">        <span class="number">0</span>: aload_0</span><br><span class="line">        1: invokespecial #1                  // Method java/lang/Object."&lt;init&gt;":()V</span><br><span class="line">        <span class="number">4</span>: <span class="keyword">return</span></span><br><span class="line">        LineNumberTable:</span><br><span class="line">        line <span class="number">3</span>: <span class="number">0</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">int</span> <span class="title">add</span><span class="params">(<span class="keyword">int</span>, <span class="keyword">int</span>)</span></span>;</span><br><span class="line">        descriptor: (II)I</span><br><span class="line">        flags: ACC_PUBLIC, ACC_STATIC</span><br><span class="line">        Code:</span><br><span class="line">        stack=<span class="number">2</span>, locals=<span class="number">3</span>, args_size=<span class="number">2</span></span><br><span class="line">        <span class="number">0</span>: iconst_0</span><br><span class="line">        <span class="number">1</span>: istore_2</span><br><span class="line">        <span class="number">2</span>: iload_0</span><br><span class="line">        <span class="number">3</span>: iload_1</span><br><span class="line">        <span class="number">4</span>: iadd</span><br><span class="line">        <span class="number">5</span>: istore_2</span><br><span class="line">        <span class="number">6</span>: iload_2</span><br><span class="line">        <span class="number">7</span>: ireturn</span><br><span class="line">        LineNumberTable:</span><br><span class="line">        line <span class="number">6</span>: <span class="number">0</span></span><br><span class="line">        line <span class="number">7</span>: <span class="number">2</span></span><br><span class="line">        line <span class="number">8</span>: <span class="number">6</span></span><br><span class="line">        &#125;</span><br><span class="line">SourceFile: <span class="string">"ByteCodeSample.java"</span></span><br></pre></td></tr></table></figure>
<p>add(1, 2)对应的局部变量表和操作数栈的交互过程：</p>
<p><img src="/xieyuanhui/2019/05/27/Java内存模型/img5.jpg" alt></p>
<p>istore：出栈        iload：入栈</p>
<p>递归为什么会引发java.lang.StackOverflowError？</p>
<p>递归过深，栈帧数超出虚拟栈深度。</p>
<p>虚拟机栈过多会引发java.lang.OutOfMemoryError异常。</p>
<h4 id="本地方法栈"><a href="#本地方法栈" class="headerlink" title="本地方法栈"></a>本地方法栈</h4><p>与虚拟机栈相似，主要作用于标注了native的方法。</p>
<h4 id="元空间（MetaSpace）与永久代（PermGen）的区别"><a href="#元空间（MetaSpace）与永久代（PermGen）的区别" class="headerlink" title="元空间（MetaSpace）与永久代（PermGen）的区别"></a>元空间（MetaSpace）与永久代（PermGen）的区别</h4><blockquote>
<p>元空间使用本地内存，而永久代使用的是jvm的内存（jdk8中永久代被元空间代替，java.lang.outOfMemoryError: PermGen space 这个错误将不复存在）</p>
</blockquote>
<p>MetaSpace相比PermGen的优势：</p>
<ul>
<li>字符串常量池存在永久代中，容易出现性能问题和内存溢出</li>
<li>类和方法的信息大小难以确定，给永久代的大小指定带来困难</li>
<li>永久代会为GC带来不必要的复杂性</li>
<li>方便HotSpot与其他JVM如Jrockit的集成</li>
</ul>
<h4 id="Java堆（Heap）"><a href="#Java堆（Heap）" class="headerlink" title="Java堆（Heap）"></a>Java堆（Heap）</h4><p><img src="/xieyuanhui/2019/05/27/Java内存模型/img6.jpg" alt></p>
<blockquote>
<p>对象实例的分配区域</p>
<p>GC管理的主要区域</p>
</blockquote>
<p>JVM三大性能调优参数 -Xms -Xmx -Xss的含义：</p>
<ul>
<li>-Xss：规定了每个线程虚拟机栈（堆栈）的大小</li>
<li>-Xms：堆的初始值</li>
<li>-Xmx：堆能达到的最大值</li>
</ul>
<p>-Xms和-Xmx一般设置成一样的，因为到Heap不够用而进行扩容时，会发生内存抖动，影响程序运行稳定性。</p>
<p>Java内存模型中堆和栈的区别—内存分配策略</p>
<ul>
<li>静态存储：编译时确定每个数据目标在运行时的存储空间需求</li>
<li>栈式存储：数据区需求在编译时未知，运行时模块入口前确定</li>
<li>堆式存储：编译时和运行时模块入口都无法确定，动态分配</li>
</ul>
<p>Java内存模型中堆和栈的区别：</p>
<p><img src="/xieyuanhui/2019/05/27/Java内存模型/img7.jpg" alt></p>
<ul>
<li>联系：引用对象、数组时，栈里定义变量保存堆中目标的首地址</li>
<li>管理方式：栈自动释放，堆需要GC</li>
<li>空间大小：栈比堆小</li>
<li>碎片相关：栈产生的碎片远小于堆</li>
<li>分配方式：栈支持静态分配和动态分配，而堆仅支持动态分配</li>
<li>效率：栈的效率比堆高</li>
<li>栈空间灵活程度不高</li>
</ul>
<p>元空间、堆、线程独占部分间的联系—-内存角度</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> com.imooc.interview.jvm.model;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">HelloWorld</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> String name;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setName</span><span class="params">(String name)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.name = name;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">sayHello</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        System.out.println(<span class="string">"Hello "</span> + name);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">int</span> a = <span class="number">1</span>;</span><br><span class="line">        HelloWorld hw = <span class="keyword">new</span> HelloWorld();</span><br><span class="line">        hw.setName(<span class="string">"test"</span>);</span><br><span class="line">        hw.sayHello();</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>元空间：</p>
<ul>
<li>Class : HelloWorld - Method : sayHello\setName\main - Field : name</li>
<li>Class: System</li>
</ul>
<p>Java堆：</p>
<ul>
<li>Object : String(“test”)</li>
<li>Object : HelloWorld</li>
</ul>
<p>线程独占：</p>
<ul>
<li>Parameter reference : “test” to String object</li>
<li>Variable reference : “hw” to HelloWorld</li>
<li>Local Variable : a with 1 , lineNo</li>
</ul>
</blockquote>
<p>不同JDK版本之间的intern()方法的区别—JDK6 VS JDK6+</p>
<p>JDK6：当调用intern方法时，如果字符串常量池先前已创建出该字符串对象，则返回池中的该字符串的引用。否则，将此字符串对象添加到字符串常量池中，并且返回该字符串对象的引用。这个版本的字符串常量池位于永久代中，容易溢出。JDK7将字符串常量池移动到了堆中。</p>
<p>JDK6+：当调用intern方法时，如果字符串常量池先前已创建该字符串对象，则返回池中的该字符串的引用。否则，如果该字符串对象已经存在于Java堆中，则将堆中此对象的引用添加到字符串常量池中，并且返回该引用；如果堆中不存在，则在池中创建该字符串对象并返回其引用。</p>
<p>例子：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> com.imooc.interview.jvm.model;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">InternDifference</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        String s = <span class="keyword">new</span> String(<span class="string">"a"</span>);</span><br><span class="line">        s.intern();</span><br><span class="line">        String s2 = <span class="string">"a"</span>;</span><br><span class="line">        System.out.println(s == s2);</span><br><span class="line"></span><br><span class="line">        String s3 = <span class="keyword">new</span> String(<span class="string">"a"</span>) + <span class="keyword">new</span> String(<span class="string">"a"</span>);</span><br><span class="line">        s3.intern();</span><br><span class="line">        String s4 = <span class="string">"aa"</span>;</span><br><span class="line">        System.out.println(s3 == s4);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>在JDK1.6版本中执行上述代码得到结果是：</p>
<blockquote>
<p>false</p>
<p>false</p>
</blockquote>
<p><img src="/xieyuanhui/2019/05/27/Java内存模型/img8.jpg" alt></p>
<p>在JDK1.6+版本中执行上述代码得到的结果是：</p>
<blockquote>
<p>false</p>
<p>true</p>
</blockquote>
<p><img src="/xieyuanhui/2019/05/27/Java内存模型/img9.jpg" alt></p>
<h3 id="参考文章"><a href="#参考文章" class="headerlink" title="参考文章"></a>参考文章</h3><p><a href="https://coding.imooc.com/class/303.html" target="_blank" rel="noopener">剑指Java面试-Offer直通车</a></p>

      
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  <p><span>本文标题:</span>Java内存模型</p>
  <p><span>文章作者:</span>xieyuanhui</p>
  <p><span>发布时间:</span>2019年05月27日 - 21:55:31</p>
  <p><span>最后更新:</span>2019年08月05日 - 15:25:17</p>
  <p><span>原始链接:</span><a href="/xieyuanhui/2019/05/27/Java内存模型/" title="Java内存模型">http://xyh5513.gitee.io/xieyuanhui/2019/05/27/Java内存模型/</a>
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